Nonwoven fabrics are used in a wide variety of applications where the engineered qualities of the fabrics can be advantageously employed. These types of fabrics differ from traditional woven or knitted fabrics in that the fibers or filaments of the fabric are integrated into a coherent web without traditional textile processes.
Filaments or fibers from which nonwoven fabrics are formed are frequently formed by spunbonding processes. In these processes, a thermoplastic polymer is melted and extruded, or “spun”, through a large number of small orifices to produce a bundle of continuous or essentially endless filaments. These filaments are cooled and drawn or attenuated and are deposited as a loose web onto a moving conveyor. The filaments are then partially bonded, such as by passing the web between a pair of heated rolls, with at least one of the rolls having a raised pattern to provide a bonding pattern in the fabric. The web of filaments can also be bonded by through-air bonding, as is known in the art.
Spunbond technology is well established in the field of nonwoven fabric production. While many advancements in the technology have been discovered, essential elements remain as described in early patents, which disclose use of a Venturi tube drawing system, including U.S. Pat. No. 3,692,618, No. 3,802,817, and No. 4,064,605, all of which are hereby incorporated by reference. As described in the basic process, a polymer, preferably a thermoplastic polymer, is melted and mixed in a extruder, with a molten polymer stream then fed, under pressure, to a spinneret assembly having a flat, machined plate defining hundreds, or thousands, of orifice openings. The polymer is forced through these openings, and emerges as a still molten, fine polymer stream. It is necessary to apply a force to the polymer stream as it cools into a filament, with such force being referred to as a drawing force. In the above-referenced patents, a Venturi tube system is used for drawing the filaments. This process requires that the multi-filament curtain of filaments be divided (usually by hand) into bundles that are fed into the mouth of a long tube, sometimes referred to as an accelerator gun. High velocity air moving through the tube accelerates the filaments, providing a positive draft relative to the speed of the filament at the spinneret face and at the point of quenching of the filament some inches below the spinneret face.
There are other techniques for providing this drawing step. It is known in the art to use Godet rolls which provide a mechanical drawing force to the extruded filaments by passing the filaments in bundles around a series of smooth metal rolls which operate at progressively increasing surface speeds. U.S. Pat. No. 4,340,563 and No. 4,405,297 describe a further advancement in the drawing of spunbond filaments, referred to as “slot draw”. In these processes, the series of drawing guns or tubes are replaced by a full-width slot which receives the entire filament curtain and maintains it. Draw tension is still provided by accelerating air, but the overall process is significantly less aggressive than the guns or Godet rolls, providing fewer processing problems as can result from filament breaks associated with other drawing methods.
The spinning of bi-component or multi-component spunbond filaments is also known in the art. U.S. Pat. No. 4,189,338 discloses a process for production of nonwoven fabric of filaments comprising polypropylene and low crystallinity polypropylene in a side-by-side configuration. U.S. Pat. No. 4,469,450 discloses bi-component filaments of polyester and polypropylene, arranged in either a side-by-side configuration, or a sheath-core configuration. U.S. Pat. No. 4,874,666 discloses a bi-component fiber with a polyester core, while U.S. Pat. No. 4,981,749 and No. 5,068,141 disclose sheath-core filaments of linear low density polyethylene and polyethylene terephthalate (PET).
Further development of bi-component (or conjugate) spinning has recognized the desirability of combining low and high melting point filaments in a fabric, such as is disclosed in U.S. Pat. No. 4,668,566, which relates to the use of a multi-beam spinning process for providing alternating polyester and polypropylene spunbond layers. U.S. Pat. No. 5,336,552 relates to a blend of olefin polymer and ethylene alkyl acrylate on one side or in the sheath, with polypropylene as the other filament component. U.S. Pat. No. 5,382,400 and No. 5,418,045 relate to the development of latent crimp in bi-component spunbond fibers. U.S. Pat. No. 5,482,772 and No. 5,512,358 relate to the formation of filaments from olefins, preferably polypropylene, with some minor polymer such as heterophasic polypropylene and butene.
Various types of apparatus for production of bi-component filaments and fibers are known in the art. U.S. Pat. No. 5,620,644 and No. 5,575,063 relate to the design of a spin pack for the melt spinning of two liquid polymer streams to produce bi-component filaments. U.S. Pat. No. 5,556,589 relates to a polymer distribution assembly and spinneret design for production of sheath-core bi-component filaments. U.S. Pat. No. 5,551,588 and No. 5,466,410, both hereby incorporated by reference, relate to a spinneret design for the production of multi-component filaments, in particular, filaments which are non-circular in cross-section, and have irregular polymer distribution. Notably, these patents disclose formation of spinneret assemblies through photo-engraving techniques, whereby the spinneret assemblies can be economically manufactured. Arrangements for diverting twin streams of dissimilar liquid phase polymers into a bi-component spinneret are known in the art, such as exemplified by U.S. Pat. No. 4,738,607, which discloses a conjugate spinning assembly having a distribution plate above the spinneret.